Due to advantages such as low cost, small size, and low power-consumption, optical transmitters based on a directly modulated laser have extensively been developed. However, a frequency chirp generated when a laser is directly modulated causes a dispersion penalty when it is transmitted over an optical dispersive medium such as an optical fiber so as to limit a transmission distance of the optical modulated signal.
To compensate the dispersion penalty, there have been provided various methods using: a dispersion-compensating optical fiber, a dispersion-compensating chirped fiber grating, an electrical equalizer, and an optical frequency reshaper.
In particular, the optical frequency reshaper-based method allows more compact and integrated design for an optical transmitter. The optical frequency reshaper may function to convert a frequency-modulated (FM) chirped signal to an amplitude-modulated (AM) signal at a transmitter side, thus requiring an accurate laser frequency control to be matched to a passband of the frequency reshaper. For example, in case where the laser frequency is switched over multiple optical communication channels, more accurate and faster control feedback mechanisms for monitoring and minimizing a frequency offset between the laser and the passband of the frequency shaper are needed.